The present invention relates to an apparatus for inspecting the internal structure of a specimen by detecting the strength of a light beam transmitted through the specimen, and more particularly to a three-dimensional structure viewer for inspecting the three-dimensional internal structure of the specimen, i.e., discovering the presence of, for example, defects in a particular cross-section thereof.
To observe the three-dimensional structure of a specimen, there has hitherto been proposed one method which uses a confocal microscope. The basic principal of this method is discussed in a document entitled "Three-Dimensional Surface Measurement Using the Confocal Scanning Microscope" by Hamilton and Wilson in Appl. Phys. B 27.211 (1982).
One example of an arrangement of the basic principle of a conventional three-dimensional structure viewer using a confocal microscope is shown in FIG. 1. In FIG. 1, a parallel light beam 2 emitted from a light source 1, e.g., a laser generator, is made to be incident upon a lens 4 through, for instance, a mirror 3 so as to cause the light beam to converge at a point 6 within a specimen 5. The light beam which is transmitted through the specimen 5 is made to converge again at the position of a pinhole 8 by a lens 7. The light beam which is transmitted through the pinhole 8 is made to be incident upon a photodetector 9, and the strength thereof is converted into an electrical signal. The electrical signal issued by the photodetector 9 is amplified by, for instance, an amplifier 10, and is then transfered as a video signal to an image display 11. Meanwhile, the specimen 5 is scanned by specimen scanners 12 and 13 with respect to the planes thereof which are parallel with the surface of the specimen 5. These scanning signals are transfered to the scanning controllers 14 and 15 of the image display 11, which in turn displays the distribution of the transmitted light beam in a case where the specimen 5 is scanned, as, for instance, monochromatic images.
In this case, the image display 11 displays the transmittance distribution within a cross-section parallel with the surface of the specimen, including a convergent point 6, by virtue of the effect of the confocal microscope using the pinhole 8. Accordingly, it becomes possible to observe the three-dimensional internal structure of the specimen 5 by varying the relative position of the convergent point inside the specimen 5 by means of a specimen scanner 16 disposed vertically with respect to the specimen's surface and by repeating the aforementioned operation. FIG. 2 shows an example of output signals of the photodetector 9 used in the apparatus shown in FIG. 1 in a case where semitransparent or opaque fine objects are present inside the specimen 5. With respect to fine objects 17 and 18 which are present in a cross-section including the convergent point of the light beam, video signals 17' and 18' corresponding to the transmittance thereof are obtained. With respect to fine objects 19, 20, 21, and 22 which are present in the other cross-sections, the strength of the transmitted light beam declines when these fine objects are located in the range of the light beam, with the result that such video signals as those shown at 19', 20', 21', and 22' are obtained. Consequently, if images are displayed using the signals of FIG. 2, there is a drawback in the sense that information on the other cross-sectional structures is superposed on the information as to fine objects 17 and 18 with respect to the desired cross-sectional structure, thereby making it difficult to effect observation of a three-dimensional structure in a true sense.